Look online for suppliers, or even a local lake. My tanks had gotten those lil' daphnia critters when I added plants from a store! My fry fed on them without me realizing they were in there and feeding my fry...

My lil' fry has a black belly from nomming on daphnia What was suggested, is get a ton of live plants from stores (or even a fresh water lake) in a bucket, they'll eat anything edible, keep the water clean of super high ammonia aaand...yeah. They breed themselves